Preparation of pickling baths



Patented Nov. 26, 1929 UNITED STATES PATENT OFFICE PAUL I. MURRILL, OFPLAINFIELD, NEW JERSEY, ASSIGNOR TO R. T. VANDERIBILT COMPANY,INCORPORATED, OF NEW YORK, N. Y., A CORPORATION OF NEW YORK PREPARATIONOF PICKLING BATHS No Drawing.

This invention relates to the preparation of a pickling bath especiallyadapted for the cleaning or pickling of iron or steel to remove scaletherefrom, although the bath may be applied more generally to thecleaning or picking of metals, such as iron or steel, for the removal ofoxides or scale therefrom.

The pickling or cleaning of iron or steel is commonly effected by meansof acid pickling or cleaning baths which remove scale or oxide from themetal. The pickling baths commonly employed are acid baths which oftenattack the metal as well as the scale. This action on the metal isundesirable and objectionable, involving not only excessive consumptionof the acids, but also corrosion or solution of the metal and tending togive a rough or pitted surface.

According to this invention an acid pickling or cleaning bath is made byadding to the acid, a mercaptothiazole and particularlyamercaptobenzothiazole in the form of a salt. In preparing the picklingor cleaning bath, mercaptothiazoles, which are insoluble in water andsparingly or slowly soluble in dilute acids, and not wetted by water ordilute acids. but soluble in alkaline solutions, such as caustic soda,are issolved in such an alkaline soution and added to the acid of thebath The acid may, and generally oes set free the mercaptothiazole fromthe solution, but in a form such that it may more readily be dissolvedin the acid of the bath than if added directly to the bath in the formof crystals or powder. The addition of a small amount of a suitablemercaptothiazole in the form of a salt. such as an alkali metal salt,prevents, retards or inhibits the attack of the acid of the bath on themetal without interfering in any substantial way with the removal of thescale or oxide by the acid bath.

The acid pickling baths with which the mercaptothiazoles are used, maybe acid baths. such as are commonly employed for pickling or cleaningiron and steel and contain, for example, about ten per cent of sulfuricacid. Such baths are gradually impoverished by the action of the acid inremoving the scale or rust and their content of ferrous sulfateincreases until the bath be- Serial No. 321,270.

comes too weak or it becomes too concentrated in ferrous sulfate forfurther use. Instead of using sulfuric acid baths, other baths may beused including acid sulfate baths.

Only a small amount of a mercaptothiazole is necessary to exert thedesired inhibiting effect on the acid of the bath. In general, only asmall fraction of one per cent is required. In the case ofmercaptobenzothiazole, for example, about two to five pounds of thecompound are sufficient for each ton of sulfuric acid used in thepickling bath. The mercaptothiazole employed should be one which isstable and does not decompose to any objectionable extent in the acidbath, and in addition, should be one which forms a soluble salt inalkaline solution.

In dissolving the mercaptothiazole in alkali prior to adding it to theacid pickling bath, sufficient alkali should be employed to dissolve thecompound, although an excess of alkali may be employed without harmfuleffect. In dissolving 2-mercaptobenzothiazole, for example, the solutionmay be prepared by dissolving 25 parts by weight of the compound inparts by weight of 10% caustic soda. This will give a solution ofapproximately the following composition:

Per cent Mercaptobenzothiazole 25.0 Sodium hydroxide 7.5 \Yater 67.5

Other mercaptothiazoles may be employed in a similar way such as2-mercapto-4-methylbenzo-thiazole, 2-mercapto-4.6-dimethylbenzo-thiazoleor 2-mercapto-5-phenylthiazole.

As it is customary to employ pickling baths atan elevated temperature,for example, around 150 to 180 F., the bath may be heated during theaddition of the mercaptothiazole inhibitor without any additionalexpense. This, however, is not necessary as the inhibitor may be addedto the bath at room temperature.

I claim:

The method of incorporating a mercaptobenzothiazole in an acid picklingor cleaning bath which comprises dissolving the mercapmamatobenzothiazole in caustic alkali to form a solution thereof in the formof the alkali metal salt and adding the resulting solution with stirrinto the acid bath whereby the 5 reaction of t e acid of the bath with thealkali metal salt of the mercaptobenzothiazole sets free themercaptobenzothiazole in a form which promotes its solution in the acidbath.

In testimony whereof I aflix my signature PAUL I. MURRILL.

